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I do at the same way (normaly i play with debuggers on pc and have not enough knowledge in java or smali but if you reading code of some easy apps then you learning by doing)
Use Software like apk editor pro (smali support)
or you can use MT-Manager pro (very nice tool)
or try dex2jar on your pc (install androidstudio with emulator or try other emulators to check results)
You can use also livedebugging on your mobile -> gameguardian, gamekiller.
Or you can use 2 mobiles: first run with apk you want debug/cheat second run with cheatengine remotemodus)
But at first try to decompile easy apks to understand what their code doing...
Greetz
1- you need luck and patience.
2- a base knowledge of any programming language (e.g : java , smali... ).
3-a base knowledge of the apk contents.
3-u need to decompile the apk using many app (eg: apk editor ,dex2jar , dex2 smali...) so u can get what they call (classes.dex) then u get the source code of the apk.
4- you need luck and patience.
5-u need to search in the java or the smali files for something that u can play with.
e.g ( if (condition == true) {activate the app} )
e.g ( money = 0; )
then u change these code to do whatever u want.
6- convert ur work to classes.dex.
7-and last but not least u need to compare the original classes.dex to the one u created and find what they call the (hex diff).
8-follow the lucky patcher custom patch guide to write these (hex diff ) to a text file and test it.
thats was the noobs way, thats why u need luck.
the pro way is something u need to search about called (live Debugging apk):
which is tracking the apk starting from any point u want to the source code.
but u need good knowledge of (low-level- programming language) in apk case its ( smali ) i think.
u can start learning by using ( apk editor.apk )its on play store witch was my way in to apk cracking, but dont try to hack big games .. just start with silly offline games so u can do something.
u r welcome.